Shaker tables are vibration tables that are used for testing various products and components. The function of a shaker table in product testing is to vibrate a product under conditions that the product might see in use except at an exaggerated rate, so that accelerated testing for product failure is possible. One type of testing is product design testing, wherein a test product is cycled under highly stressed conditions until one of the components fails. This component is then improved and the product recycled until the next component fails. This procedure is continued until maximum product design integrity is achieved. Another type of testing is product integrity testing, wherein after the product has been designed and produced, production products are tested at a lower stress level to determine if there are any structural flaws introduced in the manufactured product. This level of testing involves lower stress levels than product design testing and is generally not intended to be destructive of properly manufactured products.
In conducting product testing with a shaker table, it is generally desired to create a test pattern that includes a range of vibration frequencies within a desired test spectrum and to make these vibrations relatively uniform in amplitude and relatively uniform over the entire surface of the table. It is further considered desirable to vibrate test products with random or quasi-random vibrations in different directions at the same time.
An object in designing a shaker table is to provide a table that transmits vibrator energy efficiently to the test product without the table being overly responsive to resonant frequencies in the test range. When a table vibrates excessively at a resonant frequency in the test range, the excessive table vibration can seriously impair the objectives of the test.
To minimize the effects of resonant frequencies in the test range, some early shaker tables were designed to be as rigid as possible. As the rigidity of a table increases, the lowest or fundamental resonant frequency or natural frequency of the table increases. By making the table as rigid as possible, it was hoped to provide a fundamental resonant frequency that would be higher than the frequency range of the test (often below 2,500 Hz.). In practice, such tables still tended to produce undesirable resonant frequencies in the desired test range with the vibrators used at the time. When a rigid table produces undesirably large vibration amplitudes at the resonant frequencies, this table is considered to be too "live".
Another problem with trying to avoid resonance problems by making the table extremely rigid is that such tables also tend to be very heavy. It is desirable to be able to vibrate products with maximum amplitude and intensity with vibrators that are as small and inexpensive as possible. A heavy table has considerable mass and in effect absorbs a good deal of the vibration energy imparted by the vibrators. Thus, in order to achieve a desired vibration amplitude or acceleration level (referred to as a "g" level) with a large, solid table, large vibrators are necessary. This performance goals may exceed the capacity of pneumatic vibrators.
In an attempt to overcome these problems, Scharton U.S. Pat. No. 3,686,927 proposed the use of a light, more flexible table having a core comprising a matrix of different shaped materials with different resonant frequencies, such that the table produces multiple resonant frequency vibrations that become diffused and attenuated over the frequency range.
Baker, et al. U.S. Pat. No. 4,735,089 discloses a shaker table comprising one or more layers of a honeycomb material mounted between a solid and a segmented plate and actuated by multiple pneumatic vibrators operating at variable frequencies. Elastomeric materials are used between layers in order to dampen or reduce amplitude spikes that might occur at resonant frequencies of the table. In order to dampen resonance spikes, this table tends to absorb more vibration energy than is desirable, hence limiting the maximum performance capabilities of the table or making it necessary to use larger vibrators in order to achieve a particular amplitude or intensity of vibration in the test product.
Other manufacturers have used tables formed from solid plates with voids machined in the plates in order to provide a lighter table and more responsive table while still providing tolerably low resonant frequency vibration spikes. In such tables, multiple vibrators operating in different directions and at varying frequencies have been used to achieve multi-axis random vibrations and to avoid undesirably high resonance peaks. These tables, however, can be expensive and sometimes have problems producing low frequency vibrations.
An object of the present invention is to provide an improved vibration table that can be manufactured at low cost; transmits a high proportion of the vibration energy to the test product; transmits low frequency vibrations; produces a relatively uniform amplitude over the desired frequency range; and produces relatively uniform vibration amplitude over the surface of the table.